This invention relates to an improved annular shaft kiln for calcining solid particulate material such as limestone and in particular to a modification to the recycled gas arrangement of an annular shaft kiln. Specifically, the present invention provides a simplified arrangement for utilizing the cooling gas stream in the calcining system of a shaft kiln.
The invention is specifically directed to shaft kiln of the type disclosed in U.S. Pat. Nos. 4,025,293; 3,695,595; 3,294,936; and 4,254,221. It is to be understood however that the concepts of the present invention are equally applicable to the other types of shaft kilns.
Shaft kilns for burning or calcining solid particulate material such as limestone normally include a generally vertical hollow structure having an upper inlet for material to be burned and a lower outlet for calcined or burned material. In annular shaft kilns, an inner cylinder is positioned within the vertical structure and an annular zone is defined between the vertical structure and the inner cylinder. The top portion of the annular zone serves as a material preheater, the center section serves as a burning or calcining zone and the lower zone serves as a cooling zone. The burning zone is divided into two regions. Gases in the upper region of the burning zone flow upward, countercurrent to the flow of particulate material. Gases in the lower part of the burning zone flow downward, cocurrent with the material. The downwardly flowing spent calcining gases are recycled back to the burning zone. One or more burner chambers are defined within the annular zone, one in the upper region and one in the lower region. A draft may be drawn on the upper part of the structure so that cooling air is induced into the bottom of the structure to flow upwardly to the annular zone and cool calcined material in the lower part of the annular zone. The contact of the cooling air with the hot material results in heating of the cooling air. With prior practice, the hot spent cooling gas and recycled spent calcining gas is conducted through the inner cylinder then to an injector and from the injector to a combustion or burner chamber in the annular zone. The injectors utilize preheated combustion air under pressure to induce the flow of spent cooling air and recycled gas to enhance the draft on the vessel to draw recycled calcining gas and heated spent cooling gas to the lower burner chamber. Spent combustion air from the upper burner chamber serves to preheat solid particulate material to be burned in the upper part of the annular zone.
With the system of prior devices, and in particular with the utilization of the inner cylinder as a means for supplying preheated combustion gas to the burning chamber, there is a requirement for a complicated double wall construction of the inner cylinder since the inner cylinder is exposed to heat on both its inside and its outside. This means that the inside as well as the outside of the cylinder must be refractory lined. Further, with prior designs, the construction of the arches in the annular zone defining the burning chambers must be unduly complex in order to permit hot spent cooling gas to be withdrawn from the inner cylinder. Further, the remoteness of the hot spent cooling air from the burner injectors results in heat loss by radiation which increases fuel consumption and hence operating costs.